A substrate holder, a lithographic apparatus and method of manufacturing devices

ABSTRACT

A substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder including a main body having a main body surface; a plurality of burls projecting from the main body surface to support the substrate spaced apart from the main body surface; and a liquid control structure provided in a peripheral region of the main body surface and configured to cause liquid to preferentially flow toward the periphery of the main body surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 15200143.4 which wasfiled on 15 Dec. 2015 and which is incorporated herein in its entiretyby reference.

FIELD

The present invention relates to a substrate holder, a lithographicapparatus using the substrate holder, and a method of making devicesusing the substrate holder.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final opticalelement of the projection system and the substrate. In an embodiment,the liquid is distilled water, although another liquid can be used. Anembodiment of the invention will be described with reference to liquid.However, another fluid may be suitable, particularly a wetting fluid, anincompressible fluid and/or a fluid with higher refractive index thanair, desirably a higher refractive index than water. Fluids excludinggases are particularly desirable. The point of this is to enable imagingof smaller features since the exposure radiation will have a shorterwavelength in the liquid. (The effect of the liquid may also be regardedas increasing the effective numerical aperture (NA) of the system andalso increasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein, or a liquid with a nano-particle suspension (e.g. particleswith a maximum dimension of up to 10 nm). The suspended particles may ormay not have a similar or the same refractive index as the liquid inwhich they are suspended. Other liquids which may be suitable include ahydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueoussolution.

In a lithographic apparatus a substrate holder having a plurality ofburls to support the substrate is commonly used. The substrate isconventionally clamped to the substrate holder during exposures. In animmersion lithography apparatus, liquid can get into the space betweenthe substrate and the substrate holder during exposures. The presence ofsuch liquid can make it more difficult to unload the substrate from thesubstrate holder after it has been exposed.

SUMMARY

It is desirable, for example, to provide an improved substrate holderthat can avoid difficulties experienced in the prior art in unloading asubstrate from the substrate holder if liquid has entered the spacebetween the substrate and substrate holder.

According to an aspect of the invention, there is provided a substrateholder for use in a lithographic apparatus and configured to support asubstrate, the substrate holder comprising:

a main body having a main body surface;

a plurality of burls projecting from the main body surface to supportthe substrate spaced apart from the main body surface; and

a liquid control structure provided in a peripheral region of the mainbody surface and configured to cause liquid to preferentially flowtoward the periphery of the main body surface.

According to an aspect of the invention, there is provided a substrateholder for use in a lithographic apparatus and configured to support asubstrate, the substrate holder comprising:

a main body having a main body surface;

a plurality of burls projecting from the main body surface to supportthe substrate spaced apart from the main body surface; and

a liquid control structure provided in a peripheral region of the mainbody surface and configured to hinder movement of liquid across theperipheral region toward the centre of the main body surface, but not tohinder movement of gas across the peripheral region.

According to an aspect of the invention, there is provided a method ofmanufacturing devices using a lithographic apparatus having a substrateholder as described above and a clamp system for clamping a substrate tothe substrate holder, the method comprising:

loading the substrate onto the substrate holder;

engaging the clamp system;

exposing a pattern onto the substrate;

releasing the clamp system; and

lifting the substrate off the substrate holder.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus;

FIG. 2 schematically depicts an immersion liquid confinement structurefor use in a lithographic projection apparatus;

FIG. 3 is a side cross sectional view that schematically depicts afurther immersion liquid supply system;

FIG. 4 depicts in cross-section a substrate holder and substrate in aconventional lithographic apparatus of the localized immersion type;

FIG. 5 depicts in cross-section a part of a substrate holder accordingto an embodiment of the present invention;

FIG. 6 depicts in cross-section a part of another substrate holderaccording to an embodiment of the present invention;

FIG. 7 is a plan view of a part of a substrate holder according to anembodiment of the present invention in situ;

FIG. 8 is an enlarged perspective view of parts of channels in anembodiment of the present invention;

FIGS. 9a-9f are diagrams explaining preferential flow in channels of anembodiment of the present invention;

FIG. 10 depicts a substrate holder according to another embodiment ofthe present invention;

FIG. 11 is an enlarged view of part of the substrate holder of FIG. 10;

FIGS. 12 and 13 depict behavior of droplets of liquid on a surfacehaving regions of different contact angle;

FIGS. 14 and 15 depict behavior of a droplet on a surface having regionsof different contact angle;

FIG. 16 depicts in plan a part of a substrate holder according to anembodiment of the present invention; and

FIG. 17 depicts in cross-section a part of a substrate holder accordingto an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus of an embodimentof the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device MA in accordance with certain parameters;    -   a support table, e.g. a sensor table to support one or more        sensors or a substrate support apparatus 60 constructed to hold        a substrate (e.g. a resist-coated production substrate) W,        connected to a second positioner PW configured to accurately        position the surface of the table, for example of a substrate W,        in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising part of, one, or more dies) of the substrate W.

The illumination system IL may include various types of opticalcomponents, such as refractive, reflective, magnetic, electromagnetic,electrostatic or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure MT holds the patterning device MA. It holds thepatterning device MA in a manner that depends on the orientation of thepatterning device MA, the design of the lithographic apparatus, andother conditions, such as for example whether or not the patterningdevice MA is held in a vacuum environment. The support structure MT canuse mechanical, vacuum, electrostatic or other clamping techniques tohold the patterning device MA. The support structure MT may be a frameor a table, for example, which may be fixed or movable as required. Thesupport structure MT may ensure that the patterning device MA is at adesired position, for example with respect to the projection system PS.Any use of the terms “reticle” or “mask” herein may be consideredsynonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device MA may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the lithographic apparatus is of a transmissive type(e.g. employing a transmissive mask). Alternatively, the lithographicapparatus may be of a reflective type (e.g. employing a programmablemirror array of a type as referred to above, or employing a reflectivemask).

The lithographic apparatus may be of a type having two or more tables(or stage(s) or support(s)), e.g., two or more substrate tables or acombination of one or more substrate tables and one or more sensor ormeasurement tables configured to measure properties of the projectionsystem PS and not configured to hold a substrate W. In such “multiplestage” machines the multiple tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposure. The lithographicapparatus may have two or more patterning device tables (or stage(s) orsupport(s)) which may be used in parallel in a similar manner tosubstrate, sensor and measurement tables. The lithographic apparatus maybe of a type that has a measurement station, at which there are varioussensors for characterizing a production substrate prior to exposure andan exposure station, at which the exposures are commanded out.

The lithographic apparatus is of a type wherein at least a portion ofthe substrate W may be covered by an immersion liquid 10 having arelatively high refractive index, e.g. water such as ultra pure water(UPW), so as to fill an immersion space 11 between the projection systemPS and the substrate W. An immersion liquid 10 may also be applied toother spaces in the lithography apparatus, for example, between thepatterning device MA and the projection system PS. Immersion techniquescan be used to increase the numerical aperture of projection systems.The term “immersion” as used herein does not mean that a structure, suchas a substrate W, must be submerged in immersion liquid 10; rather“immersion” only means that an immersion liquid 10 is located betweenthe projection system PS and the substrate W during exposure. The pathof the patterned radiation beam B from the projection system PS to thesubstrate W is entirely through immersion liquid 10.

Referring to FIG. 1, the illuminator IL receives a radiation beam B froma radiation source SO. The source SO and the lithographic apparatus maybe separate entities, for example when the source SO is an excimerlaser. In such cases, the source SO is not considered to form part ofthe lithographic apparatus and the radiation beam B is passed from thesource SO to the illuminator IL with the aid of a beam delivery systemBD comprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source SO may be an integral part of thelithographic apparatus, for example when the source SO is a mercurylamp. The source SO and the illuminator IL, together with the beamdelivery system BD if required, may be referred to as a radiationsystem.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam B. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator IL can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator IL may be used to condition the radiationbeam B to have a desired uniformity and intensity distribution in itscross-section. Similar to the source SO, the illuminator IL may or maynot be considered to form part of the lithographic apparatus. Forexample, the illuminator IL may be an integral part of the lithographicapparatus or may be a separate entity from the lithographic apparatus.In the latter case, the lithographic apparatus may be configured toallow the illuminator IL to be mounted thereon. Optionally, theilluminator IL is detachable and may be separately provided (forexample, by the lithographic apparatus manufacturer or anothersupplier).

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device MA. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate support apparatus 60 can be moved accurately, e.g. so as toposition different target portions C in the path of the radiation beamB.

Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate support apparatus 60may be realized using a long-stroke module and a short-stroke module,which form part of the second positioner PW.

In the case of a stepper (as opposed to a scanner) the support structureMT may be connected to a short-stroke actuator only, or may be fixed.Patterning device MA and substrate W may be aligned using patterningdevice alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks P1, P2 as illustrated occupydedicated target portions, they may be located in spaces between targetportions C (these are known as scribe-lane alignment marks). Similarly,in situations in which more than one die is provided on the patterningdevice MA, the patterning device alignment marks M1, M2 may be locatedbetween the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate supportapparatus 60 are kept essentially stationary, while an entire patternimparted to the radiation beam B is projected onto a target portion C atone time (i.e. a single static exposure). The substrate supportapparatus 60 is then shifted in the X and/or Y direction so that adifferent target portion C can be exposed. In step mode, the maximumsize of the exposure field limits the size of the target portion Cimaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate supportapparatus 60 are scanned synchronously while a pattern imparted to theradiation beam B is projected onto a target portion C (i.e. a singledynamic exposure). The velocity and direction of the substrate supportapparatus 60 relative to the support structure MT may be determined bythe (de-)magnification and image reversal characteristics of theprojection system PS. In scan mode, the maximum size of the exposurefield limits the width (in the non-scanning direction) of the targetportion C in a single dynamic exposure, whereas the length of thescanning motion (and size of the exposure field) determines the height(in the scanning direction) of the target portion C.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratesupport apparatus 60 is moved or scanned while a pattern imparted to theradiation beam B is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate support apparatus 60 or in between successive radiation pulsesduring a scan. This mode of operation can be readily applied to masklesslithography that utilizes a programmable patterning device, such as aprogrammable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A controller 500 controls the overall operations of the lithographicapparatus and in particular performs an operation process describedfurther below. Controller 500 can be embodied as a suitably-programmedgeneral purpose computer comprising a central processing unit, volatileand non-volatile storage means, one or more input and output devicessuch as a keyboard and screen, one or more network connections and oneor more interfaces to the various parts of the lithographic apparatus.It will be appreciated that a one-to-one relationship betweencontrolling computer and lithographic apparatus is not necessary. Onecomputer can control multiple lithographic apparatuses. Multiplenetworked computers can be used to control one lithographic apparatus.The controller 500 may also be configured to control one or moreassociated process devices and substrate handling devices in a lithocellor cluster of which the lithographic apparatus forms a part. Thecontroller 500 can also be configured to be subordinate to a supervisorycontrol system of a lithocell or cluster and/or an overall controlsystem of a fab.

Arrangements for providing immersion liquid between a final opticalelement of the projection system PS and the substrate W can be classedinto three general categories. These are the bath type arrangement, theso-called localized immersion systems and the all-wet immersion systems.An embodiment of the present invention relates particularly to thelocalized immersion systems.

In an arrangement which has been proposed for a localized immersionsystem a liquid confinement structure 12 extends along at least a partof a boundary of an immersion space 11 between the final optical element100 of the projection system PS and the facing surface of the stage ortable (referred to herein as the substrate support apparatus 60) facingthe projection system PS. The facing surface of the table is referred toas such because the table is moved during use and is rarely stationary.Generally, the facing surface of the table is a surface of a substrateW, substrate table WT which surrounds the substrate W or both. Such anarrangement is illustrated in FIG. 2. The arrangement illustrated inFIG. 2 and described below may be applied to the lithography apparatusdescribed above and illustrated in FIG. 1.

FIG. 2 schematically depicts the liquid confinement structure 12. Theliquid confinement structure 12 extends along at least a part of aboundary of the immersion space 11 between the final optical element 100of the projection system PS and the substrate table WT or substrate W.In an embodiment, a seal is formed between the liquid confinementstructure 12 and the surface of the substrate W/substrate supportapparatus 60. The seal may be a contactless seal such as a gas seal 16(such a system with a gas seal is disclosed in European patentapplication publication no. EP-A-1,420,298) or an immersion liquid seal.

The liquid confinement structure 12 is configured to supply and confineimmersion liquid 10 to the immersion space 11. Immersion liquid 10 isbrought into the immersion space 11 through one of liquid openings 13.The immersion liquid 10 may be removed through another of liquidopenings 13. The immersion liquid 10 may be brought into the immersionspace 11 through at least two liquid openings 13. Which of liquidopenings 13 is used to supply immersion liquid 10 and optionally whichis used to remove immersion liquid 10 may depend on the direction ofmotion of the substrate support apparatus 60.

Immersion liquid 10 may be contained in the immersion space 11 by thegas seal 16 which, during use, is formed between the bottom of theliquid confinement structure 12 and the facing surface of the table(i.e. the surface of the substrate W and/or the surface of the substratesupport apparatus 60). The gas in the gas seal 16 is provided underpressure via gas inlet 15 to a gap between the liquid confinementstructure 12 and substrate W and/or substrate support apparatus 60. Thegas is extracted via a channel associated with gas outlet 14. Theoverpressure on the gas 15, vacuum level on the gas outlet 14 andgeometry of the gap are arranged so that there is a high-velocity gasflow inwardly that confines the immersion liquid 10. The force of thegas on the immersion liquid 10 between the liquid confinement structure12 and the substrate W and/or substrate support apparatus 60 containsthe immersion liquid 10 in the immersion space 11. A meniscus forms at aboundary of the immersion liquid 10. Such a system is disclosed inUnited States patent application publication no. US 2004-0207824. Otherimmersion liquid confinement structures 12 can be used with embodimentsof the present invention.

FIG. 3 is a side cross sectional view that depicts a further liquidsupply system or fluid handling system according to an embodiment. Thearrangement illustrated in FIG. 3 and described below may be applied tothe lithographic apparatus described above and illustrated in FIG. 1.The liquid supply system is provided with a liquid confinement structure12, which extends along at least a part of a boundary of the spacebetween the final optical element of the projection system PS and thesubstrate support apparatus 60 and/or substrate W. (Reference in thefollowing text to surface of the substrate W also refers in addition orin the alternative to a surface of the substrate support apparatus 60,unless expressly stated otherwise.)

The liquid confinement structure 12 at least partly contains immersionliquid 10 in the immersion space 11 between the final optical element ofthe projection system PS and the substrate W and/or substrate supportapparatus 60. The immersion space 11 is at least partly formed by theliquid confinement structure 12 positioned below and surrounding thefinal optical element of the projection system PS. The liquidconfinement structure 12 can comprise a main body member 153 and aporous member 183. The porous member 183 is plate shaped and has aplurality of holes 184 (i.e. openings or pores). The porous member 183can be a mesh plate wherein numerous small holes 184 are formed in amesh. Such a system is disclosed in United States patent applicationpublication no. US 2010/0045949 A1.

The main body member 153 comprises one or more supply ports 172, whichare capable of supplying the immersion liquid 10 to the immersion space11, and a recovery port 173, which is capable of recovering theimmersion liquid 10 from the immersion space 11. The one or more supplyports 172 are connected to a liquid supply apparatus 175 via apassageway 174. The liquid supply apparatus 175 is capable of supplyingthe immersion liquid 10 to the one or more supply ports 172. Theimmersion liquid 10 that is fed from the liquid supply apparatus 175 issupplied to the one or more supply ports 172 through the correspondingpassageway 174. The one or more supply ports 172 are disposed in thevicinity of the optical path at a respective prescribed position of themain body member 153 that faces the optical path. The recovery port 173is capable of recovering the immersion liquid 10 from the immersionspace 11. The recovery port 173 is connected to a liquid recoveryapparatus 180 via a passageway 179. The liquid recovery apparatus 180comprises a vacuum system and is capable of recovering the immersionliquid 10 by sucking it via the recovery port 173. The liquid recoveryapparatus 180 recovers the immersion liquid 10 recovered via therecovery port 173 through the passageway 179. The porous member 183 isdisposed in the recovery port 173.

To form the immersion space 11 with the immersion liquid 10 between theprojection system PS and the liquid confinement structure 12 on one sideand the substrate W on the other side, immersion liquid 10 is suppliedfrom the one or more supply ports 172 to the immersion space 11 and thepressure in a recovery chamber 181 in the liquid confinement structure12 is adjusted to a negative pressure so as to recover the immersionliquid 10 via the holes 184 (i.e. the recovery port 173) of the porousmember 183. Performing the liquid supply operation using the one or moresupply ports 172 and the liquid recovery operation using the porousmember 183 forms the immersion space 11 between the projection system PSand the liquid confinement structure 12 and the substrate W.

As shown in FIG. 4, a substrate W is supported on burls 20 of substrateholder WT in a recess in substrate support apparatus 60 so that theupper surface of substrate W is substantially coplanar with thesurrounding upper surface of substrate support apparatus 60. Thesubstrate W is clamped to the substrate holder WT by connecting clampopenings 89 to a clamp under-pressure so that the central space 201between the substrate W and substrate holder WT is at a lower pressurethan the space 202 above substrate W. Hence atmospheric pressure abovethe substrate W holds it firmly onto substrate holder WT. Edge seals 85a, 85 b project upwardly from the substrate holder WT to minimize thegap between the substrate holder WT and substrate W to minimize the flowof air from the surroundings into the space underneath substrate W andhence reduce the load on the clamp under-pressure. Burls 20 can beprovided in the region between edge seal ridges 85 a, 85 b; in FIG. 4two rows of burls 20 are provided but it is possible to have more, feweror no rows.

A gap 204 exists between the outer edge of substrate W and substratesupport apparatus 60, though is made as small as possible. At varioustimes in exposure of a substrate W, the substrate support apparatus 60moves under the liquid confinement system 12 so that the immersion space11 will overlap the edge of the substrate W. When it does so, immersionliquid 10 will enter the gap 204 between the edge of the substrate W andthe upper surface of the substrate support apparatus 60, as shown inFIG. 4. A first extraction opening 81, which may be two phase, isprovided and connected to a first extraction under-pressure to removeliquid which enters the gap 204. However, it is possible that liquidwill enter the peripheral space 203 between the substrate holder WT andsubstrate W at the periphery thereof. A second extraction opening 82,which may be two phase, is connected to a second extractionunder-pressure to remove liquid that has entered the space between thesubstrate W and substrate holder WT.

The present inventors have determined that in spite of the extraction ofliquid through first and second extraction openings 81, 82, after aseries of exposures it is possible that liquid will still remain betweenthe substrate W and substrate holder WT, in particular in the peripheralspace 203 outside the edge seal 85. Any liquid remaining in theperipheral space 205 hinders unloading of the substrate W. Therefore adelay is introduced before the substrate W can be lifted off thesubstrate holder WT, e.g. by e-pins (not shown), or a greater forceneeds to be exerted to lift the substrate W. Delay in releasing theunder-pressure in the central space 201 reduces throughput of theapparatus. Also, attractive forces between the liquid in the peripheralspace 203, the substrate W and the substrate holder WT tend to hold downthe edges of the substrate W as it is lifted. A tendency for the edgesof the substrate W to be held down as the substrate W is lifted causeswear of the burls of the substrate holder WT.

Accordingly, the present invention provides a substrate holder for usein a lithographic apparatus and configured to support a substrate, thesubstrate holder comprising:

a main body having a main body surface;

a plurality of burls projecting from the main body surface to supportthe substrate spaced apart from the main body; and

a liquid control structure provided in a peripheral region of the mainbody surface and configured to cause liquid to preferentially flowtoward the periphery of the main body surface.

The liquid control structure ensures that liquid that has entered intothe space between the substrate and the substrate holder tends to flowback out from that space so that it does not hinder unloading of thesubstrate. Liquid only enters the space between the substrate W andsubstrate holder WT intermittently during a series of exposures,therefore the liquid control structure does not need to completelyprevent liquid from entering the space between the substrate and thesubstrate holder but rather only needs to promote outward flow of theliquid sufficiently to ensure that the amount of liquid present when thesubstrate is unloaded is reduced. Desirably, the liquid controlstructure ensures that the rate of flow outward from the space betweenthe substrate and the substrate holder is greater than the flow inwardto the space between the substrate and the substrate holder, averagedover the time taken for exposure of a whole substrate.

In an embodiment, the liquid control structure comprises a plurality ofchannels extending from an inner side of the peripheral region to anouter side of the peripheral region. The channels encourage liquid flowtoward the outer side of the peripheral region.

In an embodiment, the liquid control structure further comprises aplurality of fins projecting from side walls of the channels and angledtowards the inner side of the peripheral region. The fins act to controlflow of liquid in the channels by ensuring that the liquid can easilyflow toward the outer side of the peripheral region but flows lesseasily towards the inner side of the peripheral region. In effect thefins act as a microfluidic diode. In an embodiment, flow of liquidtowards the center of the substrate holder is completely stopped.

In an embodiment, the channels reduce in width towards the outer side ofthe peripheral region. The reduction in width, in other words the taper,of the channels functions to create capillary forces that draw liquidtowards the outer side of the peripheral region.

In an embodiment, the channels increase in depth towards the outer sideof the peripheral region. The increasing depth of the channels towardsthe outer side of the peripheral region encourages flow of liquid towardthe outer side of the peripheral region. In an embodiment, the channelshave a depth greater than or equal to 50 μm, desirably greater than orequal to 100 μm. This depth allows a sufficient flow of liquid and caneasily be manufactured.

In an embodiment, the liquid control structure creates capillary forcesto cause liquid to preferentially flow toward the periphery of the mainbody surface. In an embodiment, the channels have a width greater thanor equal to 50 μm, desirably in the range of 60 μm to 200 μm. A width ofthese dimensions is effective to provide capillary forces. In anembodiment, the liquid control structure has a Reynolds number less than5, desirably less than 1, desirably about 0.5. A Reynolds number inthese ranges is effective to provide capillary forces. The liquidcontrol structure can be described as a microfluidic structure.

In an embodiment, the liquid control structure further comprises agroove extending along the inner side of the peripheral region. Thegroove extending along the inner side of the peripheral region allowsliquid, which may be deposited on the peripheral region in a smallregion, to spread around the peripheral region and so more of thechannels are effective to channel liquid towards the outer side of theperipheral region.

An embodiment of the invention provides a substrate holder for use in alithographic apparatus and configured to support a substrate, thesubstrate holder comprising:

a main body having a main body surface;

a plurality of burls projecting from the main body surface to supportthe substrate spaced apart from the main body surface; and

a liquid control structure provided in a peripheral region of the mainbody surface and configured to hinder movement of liquid across theperipheral region toward the centre of the main body surface, but not tohinder movement of gas across the peripheral region.

In an embodiment the liquid control structure comprises a plurality offirst regions having a first contact angle to a liquid and a pluralityof second regions having a second contact angle to the liquid that isdifferent from the first contact angle, the first and second regionsextending parallel to the periphery of the substrate holder andalternating in the radial direction. Alternating regions of differentcontract angles function to slow or stop flow of liquid inwardly of thesubstrate holder.

In an embodiment the liquid control structure comprises a plurality ofgrooves extending parallel to the periphery of the substrate holder.

In an embodiment the liquid control structure is provided on a liquidcontrol member adhered to the main body surface. In this way, the liquidcontrol structure can easily be made on a separate member which is thenattached to the main body surface.

In an embodiment the liquid control structure extends aroundsubstantially the entire periphery of the main body surface. Byproviding the liquid control structure extending around substantiallythe entire periphery of the main body surface, it is possible to ensurethat liquid is dealt with wherever on the periphery of the substrateholder it might get to. Alternatively, if it is known that in use of theapparatus liquid will only get under the substrate in limited regions ofthe periphery then it is possible to provide the liquid controlstructure at only those positions.

In an embodiment the inner side of the liquid control structure islocated within an area of the main body surface covered by a substratewhen supported by the substrate holder. If the liquid control structureextends under the substrate when supported by the substrate holder it ispossible to ensure that liquid that goes under the substrate is removed.However, it is desirable to minimize the amount of the liquid controlstructure that is underneath the substrate to minimize the area of thesubstrate that is unsupported.

In an embodiment the liquid control structure is formed of a differentmaterial from the main body, for example, the liquid control structureis formed of titanium and the main body is formed of SiSiC. If theliquid control structure is formed of a different material from the mainbody, it is possible to select a material for the liquid controlstructure in which it is easy to form structures of the necessary scalewhilst avoid compromising the material requirements for the main body ofthe substrate holder. In particular, the desired structures can easilybe formed in titanium by a variety of techniques including machining,molding or additive manufacturing.

FIG. 5 depicts a substrate holder WT forming part of substrate supportapparatus 60 according to an embodiment. Substrate holder WT has thesame shape in plan as a substrate, e.g. circular, and is substantiallythe same size as a substrate, e.g. 300 mm or 450 mm in diameter.Substrate holder WT comprises a main body 21 having a main body uppersurface 22 and a plurality of burls 20 projecting from the main bodyupper surface 22. In an embodiment, burls 20 have a height h₁ in therange of from 100 μm to 500 μm, e.g. about 150 μm. A substrate W can besupported by the distal end surfaces of burls 20, which conform to asubstantially flat support plane to support the substrate W in a flatstate. Main body 21 and burls 20 may be formed of SiSiC, a ceramicmaterial having silicon carbide (SiC) grains in a silicon matrix.

A plurality of through-holes (not shown in FIG. 5) are formed in themain body 21. Through-holes 89 allow the e-pins to project through thesubstrate holder WT to receive the substrate W and allow the spacebetween the substrate W and the substrate holder WT to be evacuated byconnection to a clamp under pressure. Other structures, e.g. to controlgas flow and/or thermal conductivity between the substrate holder WT andthe substrate W, can be provided. An edge seal 85 is provided near theperiphery of substrate holder WT. Edge seal 85 is a pair of projectingedge seal ridges 85 a, 85 b around the outside of substrate holder WT.Edge seal ridges 85 a, 85 b have a height slightly shorter, e.g. byabout 10 μm, than that of the burls 20 so that they do not contact thesubstrate W but reduce the gas flow into the space 201 between thesubstrate W and substrate holder WT so as to improve vacuum clamping.Burls 20 can be provided in the region between edge seal ridges 85 a, 85b; in FIG. 5 none are shown but it is possible to have one or more rowsof burls in this region. The substrate holder WT can also be providedwith electronic components, e.g. heaters and sensors, to control thetemperature of the substrate holder WT and substrate W.

A peripheral region 22 a of the main body upper surface 22 of substrateholder WT extends outwardly from edge seal ridge 85 b beyond thecircumference of substrate W and reaching close to first extractionopening 81. In an embodiment, the peripheral region 22 a is an annularregion having an inner diameter slightly less than the diameter of thesubstrate W and an outer diameter slightly greater than that of thesubstrate W. In an embodiment, the radial width of peripheral region 22a is a few mm, e.g. less than 10 mm. Peripheral region 22 a is providedwith a liquid control structure 200 that promotes liquid flow outwardly,i.e. towards first extraction opening 81. Liquid control structure 200is arranged so that liquid thereon flows outward preferentially, i.e. itis easier for liquid to flow outward than to flow inward.

FIG. 6 depicts an alternative arrangement of substrate support apparatus60 in which substrate holder WT is separated from the outer part ofsubstrate support apparatus 60, including first extraction channel 81 bya thermal gap 63. A small bridge 64 connects the substrate holder WT tothe outer part of substrate support apparatus 60. A liquid controlmember 220 is provided on top of the outer part of substrate supportapparatus 60 and extending across the bridge 64 to form the peripheralpart of main body surface 22. Liquid control structure 200 is providedon liquid control member 220. Bridge 64 may be omitted, so that thesubstrate holder WT is isolated from the outer part of substrate supportapparatus 60. In that case, the liquid control member 220 spans the gapbetween the substrate holder WT and the outer part of substrate supportapparatus 60 and desirably has low stiffness and low thermalconductivity.

Liquid control member 220 may be a flexible member, e.g. a sticker,which is adhered to the peripheral region 22 a. Alternatively, liquidcontrol member 220 may be a more rigid ring inset into substrate holderWT.

FIG. 7 depicts schematically a form of liquid control structure 200usable in an embodiment of the present invention. Liquid controlstructure 200 comprises a plurality of radial walls 221 provided withfins 222 and defining between them a plurality of channels 223. Fins 222are angled so as to point radially inwardly of the substrate holder WT.In FIG. 7 liquid control structure 200 is shown visible in the gap 204between the substrate W and cover ring 62. Cover ring 62 is part ofsubstrate support apparatus 60 and covers first extraction opening 81 soas to provide an upper surface coplanar with the upper surface of thesubstrate W.

FIG. 8 is an enlarged view of a part of liquid control structure 200comprising radial walls 221 and fins 222. It can be seen that fins 222are arranged in pairs extending inwardly from opposite sidewalls of eachchannel 223. For ease of manufacture, the fins 222 in all of thechannels 223 are aligned but that is not necessary.

FIGS. 9a to 9f illustrate the effect of the fins 222 on liquid seekingto flow through a channel 223. In FIGS. 9a to 9c , liquid is shownflowing in the easy wicking direction. It can be seen that the meniscus10 a of the immersion liquid 10 is always concave so that surfacetension in the meniscus 10 a generates a force that pulls immersionliquid 10 in the easy wicking direction, as indicated by the arrow. InFIG. 9a the meniscus 10 a is in the wide part of the channel 223. InFIG. 9b the meniscus 10 a is in the narrowest part of the channel 223between opposing ends of two fins 102 and in FIG. 9c the meniscus 10 ais between two rear surfaces of the fins 222. In each case the meniscus10 a is allowed to be concave.

FIGS. 9d to 9f show the stages of liquid flow in the hard wickingdirection. In FIG. 9d , the meniscus 10 a is in the wide part of thechannel 223 and is concave so that forces generated by surface tensionin the meniscus 10 a advance the liquid flow. The same applies in thesituation shown in FIG. 9e where the meniscus 10 a is on the rearsurfaces of the fins 222. However, in the situation shown in FIG. 9f ,as immersion liquid 10 begins to exit the gap between the ends of twofins 222 at the leading side, the meniscus 10 a is forced to adopt aconvex configuration. In the convex configuration, the forces generatedby surface tension in the meniscus 10 a tend to impede further advanceof the immersion liquid 10. Therefore, liquid flow in the hard wickingdirection tends to be pinned at a set of fins 222. Therefore, thestructure depicted in FIGS. 7 and 8 can be referred to as a microfluidicdiode in that it readily allows flow in one direction but resists flowin the opposite direction.

FIGS. 10 and 11 depict a substrate holder WT according to anotherembodiment of the present invention. For simplicity some details, suchas clamp openings 89, are omitted from FIG. 10. FIG. 11 is an enlargedview of a part of FIG. 10. As shown in FIG. 11, liquid control structure300 comprises a plurality of radial channels 301 provided in theperipheral region of substrate holder WT. Radial channels 301 may have awidth in the circumferential direction in the range of from 50 μm to 100μm, e.g. in the range of from 60 μm to 80 μm. The length of the radialchannels 301 in the radial direction is a few mm, e.g. less than 10 mm.The depth of the radial channels 301 in a direction perpendicular to theplane of the upper surface of substrate holder WT may be in the range offrom 50 μm to 100 μm, e.g. about 80 μm. In an embodiment, radialchannels 301 taper (i.e. reduce in width) outwardly, i.e. away from thecenter of substrate holder WT. In an embodiment, radial channels 301increase in depth towards the outside of substrate holder WT. The taperin width and increase in depth individually and in combination promoteflow of liquid outwardly, away from the center of substrate holder WT.Radial channels 301 are desirably open at their outer ends and may endat the very edge of the substrate holder WT or at a step in thesubstrate holder WT. In an embodiment, radial channels 301 end at agutter 81 a which leads immersion liquid to first extraction opening 81.The outer ends of radial channels 301 may be under the cover ring 62such as depicted in FIG. 6, when the substrate holder WT is installed inthe substrate support apparatus 60.

At the inner ends of radial channels 301, a circumferential groove 302is provided. In an embodiment circumferential groove 302 extends in acomplete loop all the way around the inner ends of radial channels 301.Circumferential groove 302 is in fluid communication with the inner endsof radial channels 301. Circumferential groove 302 may have a width inthe radial direction of between 100 μm and 500 μm. Circumferentialgroove 302 may have a depth approximately equal to the depth of channels301. Circumferential groove 302 functions to distribute liquid that mayenter under substrate W around the circumference of substrate holder WT.Since the source of liquid getting under substrate W is localized andintermittent, circumferential groove 302 assists in distributing theliquid around the peripheral region of the substrate holder WT so that alarger number of radial channels 301 can direct the liquid outwardly.

FIGS. 12 and 13 depict a further liquid control structure 400 accordingto an embodiment of the present invention. Liquid control structure 400comprises a series of alternating liquid control regions 401, 402.Liquid control regions 401, 402 are of two types: first liquid controlregions 401 have relatively low contact angle (high surface energy) tothe immersion liquid 10, whilst second liquid control regions 402 haverelatively high contact angle (low surface energy) to the immersionliquid 10. Liquid control structure 400 therefore has alternate stripesof high contact angle and low contact angle.

As depicted in FIGS. 12 and 13, liquid droplets landing on liquidcontrol structure 400 arrange themselves in droplets confined to thefirst liquid control regions 401 (low contact angle). This is showndynamically in FIGS. 14 and 15. FIG. 14 shows a droplet of immersionliquid 10 as it initially falls onto the surface in contact with firstliquid control region 401 and FIG. 15 shows how the droplet of immersionliquid 10 spreads onto and along first liquid control region 401 of lowcontact angle. If the amount of liquid flowing onto liquid controlstructure 400 is larger, the droplets may coalesce into continuouselongate regions but the liquid will prefer to flow along the firstliquid control regions 401 rather than spreading across the liquidcontrol structure 400. Therefore, the liquid control structure 400operates to hinder the flow of liquid across it. If volumes of liquidare even higher, the first liquid control regions 401 may act to pin anadvancing meniscus and thereby hinder flow of liquid across liquidcontrol structure 400.

FIG. 16 depicts liquid control structure 400 on part of a substrateholder WT. The alternating stripes of low contact angle and high contactangle are arranged as concentric rings in a peripheral region of mainbody upper surface 22 of substrate holder WT. In this configuration,liquid control structure 400 inhibits flow of liquid toward the centerof substrate holder WT. Liquid control structure 400 however does nothinder the flow of gas across the peripheral region and therefore doesnot delay the release of the under-pressure in the central regionalwhere the substrate W is to be unloaded.

The first and second liquid control regions 401, 402 of relatively lowand relatively high contact angle can be formed by various means,including chemical or mechanical treatment of an existing surface, or bydeposition of different additional materials. The regions of low contactangle (high surface energy) may have a width in the radial direction inthe range of from 50 μm to 1 mm. The regions of high contact angle (lowsurface energy) may have a width in the radial direction in the range offrom 50 μm to 1 mm.

FIG. 17 depicts in cross-section a part of a substrate holder WTaccording to another embodiment of the present invention. The substrateholder WT of FIG. 17 has a liquid control structure 400 comprising aplurality of circumferential grooves 410 provided in the peripheralregion outside of edge seal ridge 85 b. The circumferential grooves 410are concentric. Circumferential grooves 410 function similarly to thefirst liquid control regions 401 of relatively low contact angle inpromoting flow of liquid in the circumferential direction but hamperingflow of liquid radially inward of substrate holder WT. Circumferentialgrooves 410 may function as meniscus pinning features

It will be appreciated that the different liquid control structuresdescribed above can be used in combination with synergistic effect. Inparticular, use of a liquid control structure such as that describedwith reference to FIG. 16 or 17 together with a liquid control structuresuch as described with reference to FIG. 7 to 9 or 11 located radiallyoutward has particular advantage. The inner liquid control structurehinders flow of liquid toward the interior of substrate holder WT whilstthe outer liquid control structure encourages flow of liquid toward theoutside of substrate holder WT.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains one or multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 436, 405, 365, 248, 193, 157 or 126 nm).The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described.

Any controllers described herein may each or in combination be operablewhen the one or more computer programs are read by one or more computerprocessors located within at least one component of the lithographicapparatus. The controllers may each or in combination have any suitableconfiguration for receiving, processing, and sending signals. One ormore processors are configured to communicate with the at least one ofthe controllers. For example, each controller may include one or moreprocessors for executing the computer programs that includemachine-readable instructions for the methods described above. Thecontrollers may include data storage media for storing such computerprograms, and/or hardware to receive such media. So the controller(s)may operate according the machine readable instructions of one or morecomputer programs.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath, only on a localized surface area of the substrate, or isunconfined. In an unconfined arrangement, the immersion liquid may flowover the surface of the substrate and/or substrate table so thatsubstantially the entire uncovered surface of the substrate table and/orsubstrate is wetted. In such an unconfined immersion system, the liquidsupply system may not confine the immersion liquid or it may provide aproportion of immersion liquid confinement, but not substantiallycomplete confinement of the immersion liquid.

A liquid supply system as contemplated herein should be broadlyconstrued. In certain embodiments, it may be a mechanism or combinationof structures that provides an immersion liquid to a space between theprojection system and the substrate and/or substrate table. It maycomprise a combination of one or more structures, one or more fluidopenings including one or more liquid openings, one or more gas openingsor one or more openings for two phase flow. The openings may each be aninlet into the immersion space (or an outlet from a fluid handlingstructure) or an outlet out of the immersion space (or an inlet into thefluid handling structure). In an embodiment, a surface of the space maybe a portion of the substrate and/or substrate table, or a surface ofthe space may completely cover a surface of the substrate and/orsubstrate table, or the space may envelop the substrate and/or substratetable. The liquid supply system may optionally further include one ormore elements to control the position, quantity, quality, shape, flowrate or any other features of the immersion liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A substrate holder for use in a lithographic apparatus and configuredto support a substrate, the substrate holder comprising: a main bodyhaving a main body surface; a plurality of burls projecting from themain body surface to support the substrate spaced apart from the mainbody surface; and a liquid control structure provided in a peripheralregion of the main body surface and configured to cause liquid topreferentially flow toward the periphery of the main body surface,wherein the inner side of the liquid control structure is located withinan area of the main body surface covered by the substrate when supportedby the substrate holder.
 2. The substrate holder according to claim 1,wherein the liquid control structure comprises a plurality of channelsextending from an inner side of the peripheral region to an outer sideof the peripheral region.
 3. The substrate holder according to claim 2,wherein the liquid control structure further comprises a plurality offins projecting from side walls of the channels and angled towards theinner side of the peripheral region.
 4. The substrate holder accordingto claim 2, wherein the plurality of channels reduce in width towardsthe outer side of the peripheral region.
 5. The substrate holderaccording to claim 2, wherein the plurality of channels increase indepth towards the outer side of the peripheral region.
 6. The substrateholder according to claim 1, wherein the liquid control structurecreates capillary forces to cause liquid to preferentially flow towardthe periphery of the main body surface.
 7. The substrate holderaccording to claim 1, wherein the liquid control structure furthercomprises a groove extending along the inner side of the peripheralregion.
 8. A substrate holder for use in a lithographic apparatus andconfigured to support a substrate, the substrate holder comprising: amain body having a main body surface; a plurality of burls projectingfrom the main body surface to support the substrate spaced apart fromthe main body surface; and a liquid control structure provided in aperipheral region of the main body surface and configured to hindermovement of liquid across the peripheral region toward the centre of themain body surface, but not to hinder movement of gas across theperipheral region, wherein the liquid control structure comprises aplurality of first regions having a first contact angle to a liquid anda plurality of second regions having a second contact angle to theliquid that is different from the first contact angle and/or comprises aplurality of grooves extending essentially parallel to the periphery ofthe substrate holder.
 9. The substrate holder according to claim 8,wherein the liquid control structure comprises the plurality of firstregions having a first contact angle to a liquid and the plurality ofsecond regions having a second contact angle to the liquid that isdifferent from the first contact angle, the first and second regionsextending essentially parallel to the periphery of the substrate holderand alternating in a radial direction.
 10. The substrate holderaccording to claim 8, wherein the liquid control structure comprises theplurality of grooves extending essentially parallel to the periphery ofthe substrate holder.
 11. The substrate holder according to claim 8,wherein the liquid control structure is provided on a liquid controlmember adhered to the main body surface.
 12. The substrate holderaccording to claim 8, wherein the liquid control structure extendsaround substantially the entire periphery of the main body surface. 13.The substrate holder according to claim 8, wherein the inner side of theliquid control structure is located within an area of the main bodysurface covered by the substrate when supported by the substrate holder.14. A lithographic apparatus configured to project an image onto asubstrate, the lithographic apparatus comprising: a substrate holderaccording to any claim 1; and a clamp system for clamping a substrate tothe substrate holder.
 15. A method of manufacturing devices using alithographic apparatus having a substrate holder according to claim 1and a clamp system for clamping a substrate to the substrate holder, themethod comprising: loading the substrate onto the substrate holder;engaging the clamp system; exposing a pattern onto the substrate;releasing the clamp system; and lifting the substrate off the substrateholder.
 16. The substrate holder according to claim 1, wherein theliquid control structure is provided on a liquid control member adheredto the main body surface.
 17. A lithographic apparatus configured toproject an image onto a substrate, the lithographic apparatuscomprising: a substrate holder according to any claim 8; and a clampsystem for clamping a substrate to the substrate holder.
 18. Thesubstrate holder according to claim 9, wherein each of the first regionshas a different width than each of the second regions.
 19. A substrateholder for use in a lithographic apparatus and configured to support asubstrate, the substrate holder comprising: a main body having a mainbody surface; a plurality of burls projecting from the main body surfaceto support the substrate spaced apart from the main body surface; and aplurality of first regions having a first contact angle to a liquid anda plurality of second regions having a second contact angle to theliquid that is different from the first contact angle, the first andsecond regions alternating.
 20. The substrate holder according to claim19, wherein the first and second regions extend essentially parallel tothe periphery of the substrate holder and alternate in a radialdirection.